feat(stm32mp1): add regulator framework compilation

Add required macro PLAT_NB_RDEVS in platform code, and update
platform.mk to compile regulator framework.

Change-Id: I9dc7a0a4c4f5a23d9bedda368d407612c9cd21cd
Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
diff --git a/plat/st/stm32mp1/platform.mk b/plat/st/stm32mp1/platform.mk
index 3a76d28..d6fe131 100644
--- a/plat/st/stm32mp1/platform.mk
+++ b/plat/st/stm32mp1/platform.mk
@@ -200,6 +200,7 @@
 				drivers/st/iwdg/stm32_iwdg.c				\
 				drivers/st/pmic/stm32mp_pmic.c				\
 				drivers/st/pmic/stpmic1.c				\
+				drivers/st/regulator/regulator_core.c			\
 				drivers/st/reset/stm32mp1_reset.c			\
 				plat/st/common/stm32mp_dt.c				\
 				plat/st/stm32mp1/stm32mp1_dbgmcu.c			\